1. Field of the Invention
The present invention relates to a method for driving a spatial light modulator, and more specifically, to a method for driving a spatial light modulator using an alignment film with high conductivity.
2. Description of the Related Art
In the field of high definition TV having pixels arranged at a high density for displaying an image on a large image plane, various constructions have been proposed and put into practical use. Projection display apparatuses using a liquid crystal display device instead of a conventional cathode ray tube (hereinafter, referred to as CRT) have actively been developed.
In a display apparatus using a CRT, a higher density of pixels results in a lower luminance of an image plane to darken an image. Moreover, it is difficult to enlarge the size of the CRT itself. In contrast, a projection display apparatus using a liquid crystal display device operated by transistors has problems in that it is difficult to enhance the numerical aperture, i.e., the ratio of the total area of pixels with respect to the area of display, as well as the fact that a liquid crystal display device is expensive.
A liquid crystal light valve using a CRT for optical input is the focus of attention for a simple construction and for having advantages of both of the CRT and the liquid crystal display device. An example of such a device is disclosed in Japanese Laid-Open Patent Publication No. 63-109422. Today, the use of a light valve including a highly sensitive light receiving layer (namely, photoconductive layer) formed of amorphous silicon and a liquid crystal material allows a moving image to be produced on a large image plane having a size of 100 inches or larger. The use of a ferroelectric liquid crystal (hereinafter, referred to as the FLC) having a high response speed as a liquid crystal material realizes a liquid crystal light valve with a faster response and a higher resolution. Such a light valve utilizing the FLC for an excellent memory function and a high bistability thereof is considered to have a critical role in optical computing, which is a future technology for parallel operation.
A spatial light modulator (hereinafter, referred to as the SLM) including an FLC layer and a photoconductive layer is driven at a driving pulse shown in FIG. 7, which includes a reset pulse 201, a first low-voltage interval 202, a writing pulse 203, and a second low-voltage interval 204. This driving method is described, for example, in Jpn. J. Appl. Phys. 30, 3A (1991), pp. L386-L388.
According to this driving method, first, the reset pulse 201 is applied to an SLM to reset data written in an FLC layer and thus to darken the FLC, for example. That is, when the reset pulse 201 is applied, molecules of the FLC are aligned in a specified direction which prevents the FLC layer from reflecting reading light to output it from the SLM. This state (light-off state) is maintained through the first low-voltage interval 202 due to the memory function of the FLC. Then, the writing pulse 203 is applied to the SLM. At this time, when the SLM receives writing light with sufficiently high intensity, the resistance of a photoconductive layer is lowered at portions thereof exposed to the writing light. This results in the alignment of the FLC molecules being inversely changed at the corresponding portions thereof by the application of the writing pulse 203. Thus, these inversely-aligned portions of the FLC are lightened. This state (light-on state) is maintained through the second low-voltage interval 204 due to the memory function of the FLC, until the next reset pulse 201 is applied. The average intensity of the reading light for a specified time period including the repeated light-on and light-off states is recognized as the brightness of the displayed image by a viewer. In the conventional driving method, the first and the second low-voltage intervals 202 and 204 are set to be equal.
When the above SLM is used for a projection display apparatus, providing a high contrast is essential.
According to the above driving method, however, the waveform of the driving pulse needs to be symmetrical with regard to the writing pulse 203 so as to ensure that no change in the alignment of the FLC molecules with time be caused due to overcharging at an alignment film. This is the reason why the first and the second low-voltage intervals 202 and 204 are set to be equal. By this setting, the first low-voltage interval 202 occupies a large portion (approximately half) of the period of the driving pulse. This results in the effective brightness of the reading light being decreased and thus lowering the contrast.